1. Field of the Invention
The present invention relates to a magnetic recording medium.
2. Description of Related Art
Recently, digital broadcasting and data broadcasting are being spread in the television broadcasting filed, and, as magnetic recording media for audio and video signals, recording media products which can record or reproduce digital data are becoming a mainstream.
Now, it is a period of transition from analog era to digital era, and therefore a video tape recorder (VTR) or a data tape drive which can reproduce and edit both the old analog data and the new digital data is convenient, and VTR and data tape drive systems that can be used for analog and digital data are available.
Many of currently available digital VTRs and data tape drives for commercial use or for broadcasting use are designed to use a format to record digital signals and time code, i.e., time signals as well as analog audio signals simultaneously.
However, there is a difference in the frequency and depth of the recorded signal between the digital signals and the analog audio signals. Therefore, it is difficult to secure excellent properties of all the signals, and a number of vigorous studies are being made on the magnetic recording media.
For example, there is a method in which a magnetic coating composition comprising finely divided magnetic powder, which can achieve digital recording at shorter wavelengths, is applied so that the resultant magnetic layer has a thickness of 2 to 5 μn which corresponds to the depth used for analog recording.
However, this method has a problem in that the too large thickness of the magnetic layer lowers the output in electromagnetic conversion characteristics, that is, a problem of self-demagnetization due to the large thickness.
As a method for solving the above-mentioned problem, a technique is generally used in which a thick nonmagnetic layer is formed as a lower layer on the surface of the nonmagnetic support, and a magnetic layer is formed as an upper layer on the nonmagnetic layer so that the thickness of the magnetic layer is reduced to suppress demagnetization due to the large thickness, achieving a larger output. However, this method is unsuitable for analog signals since the lower layer is nonmagnetic.
On the other hand, there is a Mag-on-Mag method in which a magnetic layer suitable for analog recording is formed as a lower layer, and another magnetic layer suitable for digital recording is applied onto the lower layer to enable both analog recording and digital recording (see, for example, the below-listed Patent document 1 or 2). This technique of forming the above multi-layered magnetic layer has a feature such that the magnetic recording medium is designed so that the analog audio signals having longer wavelengths that need a recording frequency as low as about 1 KHz are recorded on the magnetic recording medium in a depth of 2 to 3 μm, and video signals having shorter wavelengths that need a recording frequency as high as several to several tens MHz are recorded on the magnetic recording medium at the surface layer in a depth of 0.3 μm or less.
Specifically, this is a method in which the upper layer is formed from magnetic powder of fine particle suitable for the higher recording frequency, the lower layer is formed from another magnetic powder having magnetic properties such that a satisfactory output can be obtained even in a longer wavelength region, and the two types of magnetic layers are stacked on one another.
See [Patent document 1]: Unexamined Japanese Patent Application Laid-Open Specification No. 53-54002 (page 2, right-hand upper column, line 10 to left-hand lower column, line 1, and FIG. 1).
See [Patent document 2]: Unexamined Japanese Patent Application Laid-Open Specification No. 3-62315 (page 2, right-hand upper column, line 8 to right-hand lower column, line 20, and FIG. 1).
Various types of Mag-on-Mag methods have conventionally been studied, and a number of studies have been made on VHS and the like in a format such that video signals are overwritten on audio signals, but, due to the above-mentioned problem of self demagnetization, for bringing the magnetic recording media produced by this method into practical use, there are restrictions of the sizes and magnetic properties of the magnetic materials used for the upper and lower layers and the thickness and construction of the individual layers.